Years ago the routes were set up and yes some people are 10 miles from no one and they get mail.   Now days to get a route extension, it must not add more than 1 mile total, which usually means 1/2 mile in each direction, but sometimes by redrawing the route you can go more than a mile if you save it back somewhere else.

PM just approved a .4 mile extension for my route and every little bit helps.